The present invention relates to an acetabular implant attached without cement.
An acetabular implant usually comprises a cup of hemispherical shape, delimiting an interior cavity, and an insert that can be fitted into this cavity. The cup is intended to be fitted into the acetabular cavity and to be attached to the pelvis, and the insert delimits a spherical cavity intended to accommodate, with a possibility of pivoting, the corresponding spherical head of a femoral shaft.
The cup is generally made of a metal, especially titanium, and the insert is made of a material which encourages the head of the femur to slide, for example an alumina ceramic or a high-density polyethylene.
Some implants are attached to the innominate bone using polymerizable synthetic cement; others are attached without cement using mechanical anchoring means such as screws.
Among these, there are so-called "press-fit" implants which are intended to be inserted forcibly into the acetabular cavity. The cup of an implant of this kind has an outside diameter which is slightly larger than that of the acetabular cavity, especially in its peripheral region, and has lumps or roughnesses which dig into the bone when the cup is impacted in the cavity.
These lumps or roughnesses are generally not enough, by themselves, to attach the cup in a way which is completely reliable over time. This is why these implants also comprise screws inserted into the bone through the cup to provide additional anchorage.
Some cups also comprise slots, and possibly openings, which give them a certain amount of radial flexibility so that they can be anchored in the acetabular cavity by expanding.
Existing implants which are attached without cement anchor the cup securely and reliably in the acetabulum, but do, however, have the major drawback of allowing debris from the wearing of the material of which the insert is made to diffuse into the acetabular cavity and into the body.
This debris is the result of the rubbing of the head of the femur in the insert and of micro-movements of the insert within the cup. It is also the result of the rubbing of the insert against the somewhat sharp edges which delimit the aforementioned recesses, holes, slots or openings in the cup, this being exacerbated by the fact that the insert tends to creep into these recesses, holes, slots or openings under the effect of the repeat stresses to which the prosthesis is subjected.
Furthermore, the aforementioned holes, slots or openings allow particle-laden synovial fluid to flow towards the bone, this flow being the result of a "pumping effect" caused by the successive application and release of pressure on the implant in the cavity as the patient walks.
This polyethylene debris causes osteolysis, which is detrimental to the firm anchorage of the screws over time, and is not well tolerated by the body.
Another drawback of "press-fit" cups is the risk of an inadequate bond between the external surface of the cup and the ilium, especially at the catching reliefs in the equatorial region of the cup, which means that it is often necessary to use screws for fastening into the bone tissue, even in the case of cups which are coated with a material which assists with osteo-integration, such as calcium hydroxyapatite.